1. Field of the Invention
This invention relates to a printed-wiring board and particularly a structure of a pad portion which is loaded with a surface mounted part.
2. Disclosure of the Prior Art
In the past, there has been a printed-wiring board having pads 2 for being loaded with surface mounted parts on the surface of an insulating substrate 1, as shown in FIG. 1(a), wherein the side walls of the pads have an exposed form and the section thereof has a trapezoid or a form similar to that. Such printed-wiring board is obtained by a technique which comprises photoetching the insulating substrate 1 on which a conductive foil is spreaded and fixed or a pattern plating technique which comprises applying a heavy gage copper plating on the insulating substrate 1, on which a conductive foil is spreaded and fixed, in the form of a desired pattern and thereafter carrying out a quick etching to obtain the desired pattern, or the like. The thus obtained printed-wiring board is widely used in various industrial field.
Moreover, the above printed-wiring board can be generally produced according to, for example, a process as shown in FIGS. 2(a) to (f). First, as shown in FIG. 2(a), a hole for forming a through-hole portion is mounted in an insulating substrate 1 with copper foils 12 being spreaded and fixed on both main surfaces of inside and outside thereof; as shown in FIG. 2(b), a panel copper plating layer 13 is formed on an inner wall of the hole for forming the through-hole portion and on the main surfaces of the copper foils 12; as shown in FIG. 2(c), an etching resist layer 14 is formed in a desired pattern; as shown in FIG. 2(d), an etching treatment is made by using the etching resist layer 14 as an etching mask to obtain a desired circuit pattern 13a. In addition, as shown in FIG. 2(e), the etching resist layer 14 is stripped off and removed; and as shown in FIG. 2(f), a soldering resist layer 16 is formed.
In the prior printed-wiring board as mentioned above, as shown in FIG. 1(b), a conductive material at the side walls of the pads 2 for being loaded with the surface mounted part remains exposed. Thus, in the case that the pads 2 are bonded to a lead 4 of a part by a soldering process of a reflow system wherein cream solder is used, there is a defect that solder 5 is adhered to an exposed conductive surface of the sides of the pads 2 and as result soldering bridge takes place between adjacent pads due to soldering balls 5a.